This project continues an innovative line of research on how to optimally use sleep as an intervention to promote cognitive recovery from, and resistance to, the neurobehavioral risks posed by chronic partial sleep deprivation. Chronic insufficient sleep is estimated to affect at least 20 percent of adults. It can result from medical conditions and sleep disorders, as well as work demands, and social or domestic responsibilities. It is associated with significant clinical morbidity, and directly causes errors and accidents that are due to its adverse neurobehavioral effects on alertness, mood, and cognitive functions. In seminal experiments conducted under this grant, we showed that the neurobehavioral effects of chronic sleep restriction accumulate to severe levels in a few days, without the full awareness of the affected individuals, and that recovery from chronic sleep restriction requires more sleep than previously assumed. We also discovered that recovery from chronic sleep was illusory, because it masked a heightened neurobehavioral vulnerability to even a single post-recovery night of sleep restriction. The implications of these findings are that apparent recovery from chronic sleep restriction masks a more severe cognitive response to subsequent sleep restriction suggesting that there are longer time constants in the brain for neurobehavioral recovery from chronic sleep restriction. In light of this finding, we now seek to determine whether additional nights of extended recovery sleep will reduce the heightened vulnerability induced by prior exposure to sleep restriction. A total of 110 adults will be studied in the laboratory during a 17-day protocol evaluating cognitive, psychological and physiological responses to varying recovery days between two sleep-restriction periods. The results will establish the number of nights of recovery sleep needed to prevent accelerated deterioration during a subsequent period of sleep restriction. The findings will advance theoretical understanding of sleep homeostasis and its relationship to cognitive functions, as well as inform theories of sleep need, and have substantial implications for sleep biology, for the treatment of clinical disorders that regularly disrupt sleep, and for managing lifestyle factors that frequently restrict sleep.